1. Field of the Invention
The present invention relates to improvements to communications systems. More particularly, the present invention relates to wireless discrete multitone spread spectrum communications systems.
2. Description of the Related Art
Wireless communications systems, such as cellular and personal communications systems, operate over limited spectral bandwidths and must make highly efficient use of the scarce bandwidth resource for providing good service to a large population of users. A Code Division Multiple Access (CDMA) protocol has been used by wireless communications systems for efficiently making use of limited bandwidths and uses a unique code for distinguishing each user""s data signal from data signals of other users. Knowledge of the unique code with which any specific information is transmitted permits separation and reconstruction of each user""s message at the receiving end of the communication channel.
Adaptive beamforming technology has become a promising technology for wireless service providers for offering large coverage, high capacity, and high quality service. Based on this technology, a wireless communication system can improve its coverage capability, system capacity, and performance significantly. A personal wireless access network (PWAN) system, described in the cross-referenced Alamouti, Stolarz, et al. patent applications, uses adaptive beamforming combined with a form of the CDMA protocol known as discrete multitone spread spectrum (DMT-SS) for providing efficient communications between a base station and a plurality of remote units (RUs).
The remote units are powered primarily from AC power sources and include a battery for providing battery backup power when AC power fails. To conserve battery power, an RU has a sleep mode of operation with periodic power-up modes for checking whether any calls are attempting to be connected to the RU. When an RU is in a sleep mode, it expedient that the system operate in such a way so that appropriate actions are taken for completing a call to a sleep mode RU.
One approach for ensuring that calls are completed to a remote unit operating in a sleep mode is to maintain a database at a central location that stores the current operating mode of each remote in the system. When a remote unit enters a sleep mode of operation, the remote unit reports the change of operational status to the database. Similarly, the remote unit reports a change of status back to a standby operating mode. This approach has a drawback when a number of remote units recorded in the database experience frequent power outages. In such a situation, recording, managing and synchronizing power outage information in the database is particularly cumbersome when the database is large, perhaps holding status information for 3 to 4 thousand remote units. This drawback is further compounded when the database is duplicated multiple times throughout the system. When several thousand subscribers experience a power outage and AC power is restored before the database has completed recording the power outage, a database approach becomes unwieldy. Another complicated situation is when multiple remote units lose power at the same time. The affected remote units cannot all access the channel simultaneously for communicating their status to the database. A collision avoidance scheme must be implemented that spans a period of time and that is open for the possibility of power being restored before the database has been completely revised.
This approach has another drawback in that a remote unit entering the sleep mode consumes system bandwidth in notifying the database. FIG. 4 shows an exemplary flow of internal messaging that occurs between various layers of a remote unit when loss of AC power is detected and a database is notified of the operational status change. Time is shown along the vertical axes of FIG. 4, with advancing time being indicated toward the bottom of FIG. 4. In FIG. 4, four layers of the remote unit operating system are shown: Health; OAMandP (Operations, Administration, Maintenance and Provisioning), MAC (Media Access Control) and physical. Only MAC layer of the base station is shown. At 40, AC power failure is detected by the Health layer. At 41, an EVENT message is sent from the Health layer to the OAMandP layer indicating that AC power has failed. The OAMandP layer sends an ACTION message to the MAC layer at 42. The MAC layer responds at 43 by sending an ACTION_RSP message to the OAMandP layer indicating that base station notification is pending. At 44, the MAC layer waits a random length period of time before sending an unsolicited CAC message at 45 to the MAC layer of the base station indicating the need for the remote unit to enter the sleep mode. At 46, the MAC layer of the base station sends an acknowledgment message to the MAC layer of the remote unit acknowledging receipt of the unsolicited CAC message. In response, the MAC layer of the remote unit sends an EVENT message at 47 to the OAMandP layer that the notification is done. The OAMandP layer first sends an EVENT message to the MAC layer indicating that the sleep mode has been entered at 48, and then sends a message at 49 to the physical layer to power down.
What is needed is a way for a PWAN system to be aware that a remote unit is operating in a sleep mode so that appropriate actions can be taken by the system so that calls can be completed to a remote unit operating in a sleep mode.
The present invention provides a method for reducing power consumption of a remote unit in a PWAN system. A remote unit is powered using a battery backup power supply when an AC power supply fails at the remote unit. A sleep mode of operation is entered at the remote unit that has a reduced power consumption for the battery backup power supply. The remote unit is synchronized to a TDD timing structure a predetermined period of time after entering the sleep mode of operation. A standby mode of operation is then entered at the remote unit in which a CONNECT message indicating an incoming call for the remote unit is scanned for by the receiver. When no CONNECT message is received, the remote unit reenters the sleep mode of operation. According to the invention, the predetermined period of time is a predetermined number of subframes after a boundary subframe of the TDD timing structure. Preferably, the predetermined number of subframes is based on an identification number of the remote unit.
The present invention also provides a remote unit for a personal wireless area network that includes a receiver, an AC power supply, a battery-backup power supply and a controller. The battery-backup becomes operative when the AC power supply fails and supplies power to the receiver. The controller detects when the AC power supply fails and controls the receiver and the battery-backup power supply by invoking a sleep mode of operation. The sleep mode of operation is periodically interrupted by the controller controlling the receiver and the battery-backup power supply to enter a standby mode of operation in which the receiver scans a CONNECT message indicating an incoming call. The controller coordinates the sleep mode and the standby mode of operations based on a frame count that is generated from an identification number of the remote unit.
In accordance with another aspect of the invention, a highly bandwidth-efficient communications method is disclosed for the base station to enable it to communicate with a remote unit that is in the sleep mode. The remote unit has a unique identification value that is different from the identification value of other remote units that may be communicating with the base station. The base station begins by establishing a periodic reference instant at the base station and at the remote station. Then the base station determines a delay interval following the periodic reference instant at the base station, the delay interval being derived from the unique identification value of the remote unit. The base station receives spread signals from the remote units with which it communicates, each comprising an incoming data traffic signal spread over a plurality of discrete traffic frequencies. The base station adaptively despreads the signals received it receives by using despreading weights. The base station attempts to initiate a communication with the remote unit that is currently in the sleep mode. If the attempting step fails to initiate communications with the remote unit, the base station concludes that the remote unit is in the sleep mode. In response to this, the base station waits for the delay interval following the periodic reference instant at the base station before transmitting to the remote unit. The base station then transmits to the remote unit a spread signal comprising an outgoing data traffic signal spread over a plurality of discrete traffic frequencies. The remote unit has simultaneously changed from the sleep mode to the standby mode and is able to receive and respond to the spread signal transmitted from the base station.
In accordance additional aspects of the invention, the base station is part of a wireless discrete multitone spread spectrum communications system. Further, the periodic reference instant is established by a beginning subframe count instant that is incremented by a packet count value at the base station and at the remote unit. In addition, the delay interval is determined by a value N of a quantity of M least significant bits of the unique identification value of the remote unit, the delay interval being an interval required for the occurrence of a plurality of N of the beginning subframe count instants. The resulting invention enables the base station to be aware that a remote unit is operating in a sleep mode so that appropriate actions can be taken by the base station to assure that calls can be completed to the remote unit.